Allowing themes to expand the list is on my TODO for this. Basically, I
was thinking that the config dialog would list all the "standardized"
color classes, as it does now, then at the bottom of the list, have it
show any "extra" color classes that the theme supports, which would
include module color classes.
I'm completely open to input on this, and now is the time as the dialog
is in it's infancy.

Originally, the idea behind the color classes was not to provide an
exhaustive directory of parts that can have their colors overridden.
It was more that you would provide a limited set of classes so the
user can easily set the color scheme for their whole environment.
Maybe the better solution is hierarchical color classes, so the basic
list would affect most elements, but individual parts could be changed
in advanced mode.